Most integrated circuits (ICs) and other miniature components such as oscillators, magnetic sensors, etc. are packaged in plastic or ceramic packages with solder pads or with extending metal leads for soldering to a printed circuit board or for insertion into a socket. Piezoelectric devices are common, and include electromechanical resonators, filters, sensors, and actuators. Electronic filters may be made with one or more mechanical resonators. Mechanical resonators are a key element of oscillator circuits, for example, and may be a quartz crystal, a piezoelectric crystal, an aluminum nitride bulk acoustic resonator, or a surface acoustic wave device. In most cases, a single package will only contain a single circuit component, although multiple IC chips are more commonly being manufactured within a single package. The use of such packages results in a low circuit density as the ceramic or plastic package consumes relatively large areas of the mounting surface, particularly if a socket is used.
Some of the primary considerations in developing improved high-density electronic packaging modules are:
Optimizing packing density to achieve the lowest possible volume per element, essentially the smallest module that is production worthy.
Eliminating packaging elements where possible, such as ceramic and plastic boxes and printed circuit boards, flex circuits and other substrates.
Simplifying the fabrication procedures.
Enhancing the structural strength of the elements.
Improving reliability of the electronics and of the techniques for interconnecting the electronics with the electrical leads.
Maximizing suitability for test and repair at the lowest level of assembly and throughout assembly.
Minimizing the cost per element.
A significant deterrent to reducing microcircuitry size is the electromechanical resonator. Companies, such as Statek Corporation, Orange, Calif., produce small crystals, which are one form of electromechanical resonator. The mechanical resonator packages themselves, which may be surface mounted or leaded, are relatively large. The electromechanical resonator package is generally a package comprised of metal, glass and/or ceramic that contains the crystal resonator, inverter, active element transistors, resistors, capacitors and/or inductors.
Microstimulators, are exemplified by the devices and systems described in U.S. Pat. Nos. 6,164,284, 6,185,452, 6,208,894, 6,472,991, and 6,315,721, which are incorporated herein by reference in their entirety. Microstimulators are typically elongated devices with metallic electrodes at each end that deliver electrical current to the immediately surrounding living tissue. One significant characteristic of these microstimulators is that they eliminate the need for electrical lead wires. The microelectronic circuitry and inductive coils that control the electrical current applied to the electrodes are protected from the body fluids by a hermetically sealed capsule. This capsule is typically made of rigid dielectric materials, such as glass or ceramic, that transmits magnetic fields but is impermeable to water, as well as biocompatible materials such as titanium.
An implantable miniature stimulator represents a typical application for a microstimulator. U.S. patent application Ser. No. 10/280,841, incorporated herein by reference in its entirety, presents the state of the art for crystal oscillators in these microstimulator devices. Because the microstimulator is small and leadless, it may be advantageously placed anywhere in the body of a human. Typical dimensions for this device are about 5 to 60 mm in length and about 1 to 6 mm in diameter. Obviously, the smaller the microstimulator, the more readily it may be placed in living tissue. Therefore, there is a need to reduce the size of the oscillator to enable production of smaller microcircuitry and hence smaller devices.